Actually looks pretty clean to me! Why do you say it makes them unusable?
That is a very stretched picture and the actual pixel ADU values are very low. Every camera I own shows similar patterns, thats why you take Bias frames. Your light frames will have similar low level patterns so subtracting the bias removes them from the lights.
You need to stack a load of bias frames together to make a master bias, that will look cleaner still but still have the vertical pattern. How many you stack is up to you. I do 100!